Our major focus is now on the microtubule cytoskeleton of a Trypanosomatid. Crithidia fasciculata presents many unusual features, perhaps most conspicuously a paraflagellar rod linked to doublets 4 to 7, and a regularly spaced subpellicular corset of microtubules linked to each other and to the plasma membrane. To elucidate the structure and regulation of these links, we have begun to characterize microtubule-associated proteins. We find a 42-kDa polypeptide in isolated pellicles, which does not appear to bind an antibody which recognizes actin in other protozoa. Taxal MAPs from cytoplasmic tubulin include 4 prominent heat-stable polypeptides (27-125 kDa) and at least 5 minor high molecular weight compounds. Second we have resumed the study of tubulin modification by reversible C-terminal tyrosine addition in brain, and extended this also to Crithidia. The brain carboxypeptidase that releases tyrosine has been pruified 2000-fold using an FPLC pH gradient column. Despite the presence of a specific tyrosine adding enzyme in Crithidia, cytoplasmic and pellicular tubulins have been isolated devoid of C-terminal tyrosine. Flagellar tubulin, however, is partially tyrosinated, and is further distinguished by the presence of some "non-substrate" speciec, as in mammalian tubulins.